Non-flammable and non-combustible adhesive bonding systems having adherence to low energy surfaces

ABSTRACT

This invention relates to (meth)acrylate based polymerizable compositions and multipart adhesive systems prepared therefrom, which include an aziridine-containing compound in a carrier material with which the aziridine-containing compound is unreactive. The inventive composition demonstrates a flash point above 140° F., desirably above 200° F., for each of the parts of the adhesive system. To achieve the physical properties of the inventive compositions and adhesive systems, it is desirable for the carrier material to have such flash points; however, provided that in their entirety the inventive compositions and adhesive systems have such flash points, it is not necessary for the carrier material itself to have such flash points. The inventive compositions and adhesive systems also include a package for controlling the rate of cure and indicating the extent of cure. The inventive compositions and adhesive systems are particularly well suited to bonding applications which involve at least one low energy bonding surface, for example, polyolefin, polyethylene, and polypropylene surfaces.

This Application claims benefit of Provisional Application No. 60/390,423 filed Jun. 24, 2000 and Provisional Application No. 60/419,999 filed Oct. 22, 2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to (meth)acrylate based polymerizable compositions and multipart adhesive systems prepared therefrom, which include an aziridine-containing compound in a carrier material with which the aziridine-containing compound is unreactive. The inventive compositions demonstrate a flash point above 140° F., desirably above 200° F., for each of the parts of the adhesive system. The inventive compositions and adhesive systems also include a package for controlling the rate of cure and indicating the extent of cure. The inventive compositions and adhesive systems are particularly well suited to bonding applications which involve at least one low energy bonding surface, for example, polyolefin, polyethylene, and polypropylene surfaces.

2. Brief Description of Related Technology

Low energy surfaces such as polyolefins, i.e., polyethylene, polypropylene, polybutene, polyisoprene, and copolymers thereof, are well known to be difficult to bond to each other and to other surfaces using adhesive bonding technology since they have few active bonding sites available at the free surfaces. Low energy surfaces are defined as those having surface energies less than 45 mJ/m², more typically less than 40 mJ/m², or less than 35 mJ/m².

Bonding low energy surfaces by surface pre-treatments such as flame treatments, plasma treatments, oxidation, sputter etching, corona discharge, or primer treatments with a high surface energy material is well known. Such treatments disrupt the bonds at the surface of the low energy material providing sites which are reactive and which can participate in bonding reactions with adhesive materials. However, such surface pre-treatments are usually undesirable, in that they add cost to the process, they are not particularly reproducible in their results, and the effect of the pre-treatments wears off with time so the pre-treated surfaces must be re-pre-treated if they are not bonded within a reasonable period of time.

The chemistry of organic boron compounds has been studied in detail [see e.g. D. Barton and W. D. Ollis, “Comprehensive Organic Chemistry”, Vol. 3, Part 14, Pergamon Press (1979) and H. C. Brown, Boranes in Organic Chemistry, Cornell University Press, Ithaca, N.Y. (1972)]. The use of organoboranes such as the trialkylboranes including triethylborane and tributylborane for initiating and catalyzing the polymerization of vinyl monomers is well known. However, such organoborane compounds are known to be flammable in air so that the compounds and compositions containing them require special handling and the compositions have poor shelf stability [see e.g. U.S. Pat. No. 3,236,823 (Jennes), and the Background section of U.S. Pat. No. 5,935,711 (Pocius), at col. 2].

Certain boron alkyl compounds and their use as initiators of polymerization are described in U.S. Pat. Nos. 4,515,724, 4,638,092, 4,638,498, 4,676,858 and 4,921,921 (the “Ritter patents”).

U.S. Pat. Nos. 5,106,928, 5,143,884, 5,286,821, 5,310,835 and 5,376,746 disclose a two-part initiator system for acrylic adhesive compositions, in which the first part includes a reportedly stable organoborane amine complex and the second part includes a destabilizer or activator such as an organic acid or an aldehyde.

Japanese Patent Publication No. S48-18928 describes a method for adhering polyolefin or vinyl polymers using an adhesive obtained by adding trialkylboron to a vinyl monomer, with or without vinyl polymer. Examples of trialkylboron include triisopropylboron, tri-n-butylboron, tripropylboron and tri-tert-butylboron.

U.S. Pat. No. 3,275,611 (Mottus) describes a process for polymerizing unsaturated monomers with a catalyst comprising an organoboron compound, a peroxygen compound and an amine complexing agent for the boron compound.

It is well known that the bonding of polyolefin substrates and other low surface energy substrates causes particular difficulties. Attempts have been made to overcome these difficulties by the extensive and expensive substrate surface preparation described above, or by priming the surface with a high surface energy primer. However, it is desired to develop adhesive compositions, which will bond low surface energy substrates without such surface preparation.

U.S. Pat. No. 5,539,070 (Zharov), and U.S. Pat. Nos. 5,616,796, 5,621,143, 5,681,910, 5,684,102, 5,686,544, 5,718,977, 5,795,657 and the '711 patent describe organoborane amine complexes which can be used in systems that initiate the polymerization of acrylic monomers in compositions useful for bonding low surface energy plastics substrates such as polyethylene, polypropylene and polytetrafluoroethylene.

International Patent Publication No. WO 99/64528 describes low odor polymerizable compositions comprising monomer blends and organoborane amine complex initiators. These systems require the preparation of trialkyl borane amine complexes to achieve the desired performance and shelf stability. The manufacture of such complexes is an undesirably complicated process. Further, the presence of the amines results in cured adhesives that have a tendency to become yellow in color on aging.

International Patent Publication No. WO 01/44311 also describes amine organoborane complex polymerization initiators in bonding compositions for low surface energy substrates.

International Patent Publication No. WO 01/32716 (“PCT '716”) acknowledges that while complexes of an organoborane and an amine may be useful in many applications, certain problems may arise due to the use of amine complexing agents in such conventional complexes. For example, when the complexes contain a primary amine, adhesives prepared therefrom may be prone to discoloration, such as yellowing; further, when including reactive diluents, such as aziridines described in International Patent Publication No. Wo 98/17694, for example, in compositions containing the complexes, the diluents may prematurely react with protic amines (i.e., those amines in which a nitrogen atom is bonded to at least one hydrogen atom) in such complexes, prematurely decomplexing the organoborane initiator.

PCT '716 therefore proposes a complex of an organoborane and a complexing agent of at least one hydroxide or alkoxide, particularly a complex represented by the formula

-   -   where R¹ is an alkyl group having 1 to about 10 carbon atoms; R²         and R³ may be the same or different and are selected from alkyl         groups having 1 to about 10 carbon atoms and phenyl-containing         groups; the value of “v” is selected so as to provide an         effective ratio of oxygen atoms of the alkoxides and/or         hydroxides to boron atoms in the complex; each R⁴ is         independently selected from hydrogen or an organic group (e.g.,         an alkyl or alkylene group); M^((m+)) represents a countercation         [comprising a monovalent cation, such as a Group IA metal (e.g.,         lithium, sodium and potassium) cation or onium, or a multivalent         cation, such as a Group IIA metal (e.g., calcium and magnesium)         cation]; n is an integer greater than zero; and m is an integer         greater than zero. Particular complexing agents are stated to         have a countercation selected from sodium, potassium and         tetraalkylammoniums. When any R⁴ is hydrogen, the complexing         agent is said to comprise at least one hydroxide. When any R⁴ is         an organic group, the complexing agent is said to comprise at         least one alkoxide.

According to PCT '716, the complexing agent (i.e., the hydroxide or alkoxide) is used in the form of a salt. That is, the complexing agent is stabilized by a suitable countercation such that the complexing agent is capable of complexing the initiator. Thus, in the Formula, M^((m+)) represents a countercation that stabilizes the complexing agent, not a cation that forms an ionic compound with the organoborane initiator.

PCT '716 states that hydroxides and alkoxides provide strong coupling to organometallic initiators, such as organoboranes, with the resulting complexes having excellent oxidative stability. Thus, complexing agents of at least one hydroxide, alkoxide, or mixtures thereof are said to be particularly beneficial. Such a complex is stated to be a tightly co-ordinated salt formed by association of a Lewis acid (the initiator) and a Lewis base (the hydroxide or alkoxide). This indicates that the oxygen atom of the alkoxide or hydroxide is bonded or co-ordinated to the boron atom of the initiator.

Tetraorganylborate salts of tetraalkylammonium, sodium or lithium are known as photoinitiators in photocurable compositions for imaging materials (see e.g. U.S. Pat. Nos. 4,950,581, 6,110,987 and 6,171,700). Tetraorganylborate salts do not however have a boron-hydrogen bond. The distinction between boron-hydrogen compounds, triorganylboranes and organoborate salts is well illustrated in D. Barton and W. D. Ollis, “Comprehensive Organic Chemistry”, in which Chapters 14.2, 14.3 and 14.4 of Vol. 3 are devoted to them. And the photopolymerizable compositions of the '581, '987 and '700 patents are not intended for use as adhesives, sealants and the like.

Moreover, it is not predictable whether photopolymerization initiators will be suitable for other cure systems or for bonding substrates having particular surface energy conditions.

In designing and developing a product which is to be transported, one must be cognizant of the transportation regulations of the various countries to which, through which, and/or from which the product will be transported. Thus, to the extent that a product contains a flammable or combustible constituent, a product developer should consider whether and to what extent the ability of the product to be shipped may be impaired, such as through special packaging requirements or particular modes of transportation. To this end, flammability is often considered in connection with a flash point of less than 140° F. and combustibility is often considered in connection with a flash point of less than 200° F.

Despite the work of many in this field, there is a need for polymerizable compositions for bonding low surface energy substrates, choices among such as polyolefins, and for end users to have a variety of such compositions which achieve that result through different technical strategies, and for such compositions to be transportable without excessive restrictions due to flammability or combustibility.

SUMMARY OF THE INVENTION

The present invention which is directed to a polymerizable composition, which includes a free radical polymerizable component, such as one based on (meth)acrylates, an alkylated borohydride as defined below, an aziridine-containing compound, and a carrier therefor which is unreactive toward the alkylated borohydride. The inventive compositions demonstrate a flash point greater than 140° F. (60° C.) and desirably greater than 200° F. (93.3° C.).

In addition, another aspect of the invention provides an indicator system which controls the rate of cure by the addition of a halogen solution, such as an iodine solution, and which also serves as an indicator for when cure begins.

DETAILED DESCRIPTION OF THE INVENTION

For purposes of the present invention, the terms (meth)acrylate and (meth)acrylic are used synonymously with regard to the monomer and monomer-containing component. The terms (meth)acrylate and (meth)acrylic include acrylate, methacrylate, acrylic, and methacrylic.

And, the term “non-flammable” will mean a flash point of greater than about 200° F. (94° C.), whereby heating is required for the material to catch fire; and the term “flammable” will mean the material is ignitable with a spark rather than by heating, and having a flash point of about less than 100° F. (38° C.). Generally, compounds which have a flash point between 140° F.-200° F. (60° C.-93° C.) are classified as “combustible”. These above definitions are believed to be generally accepted ones in accordance with those set by the U.S. Department of Transportation (“DOT”).

The present invention relates to polymerizable (meth)acrylate compositions and adhesive systems based on such (meth)acrylate compositions, which when applied onto a substrate results in the formation of a polymeric adhesive material which strongly adheres to surfaces, particularly low energy surfaces such as polyolefins.

The polymerization reaction promoted by the initiator systems also causes bonding of the adhesive to the surfaces being bonded. The speed and extent of the polymerization can be controlled by varying the ratio of the various constituents. The multi-part adhesive system of the present invention has shown average bond strengths to low energy surface materials such as polyethylene and polypropylene in excess of about 1,000 psi.

And in another aspect of the invention, the rate of cure may be controlled by the addition of a halogen solution, such as an iodine solution, which also serves as an indicator for when cure begins.

The adhesive compositions disclosed herein are useful for bonding a large range of substrates including metals, plastics and glass to similar or different substrates and perform better than most previously available two-part (meth)acrylate based adhesives on many of these substrates, especially low energy surfaces having a surface energy of less than 45 mJ/m². These low energy surfaces include polyolefins, for example, polyethylene and polypropylene, acrylonitrile-butadiene-styrene and polytetrafluroethylene, or relatively low surface energy substrates such as polycarbonate, to similar substrates, to each other, or to different substrates including metals, other plastics and glass.

Also disclosed herein are kits comprising the (meth)acrylate compositions for use as adhesive systems, the resultant bonded assembly using the (meth)acrylate based adhesive system of the present invention, and a method of bonding low energy substrates with the (meth)acrylate based adhesive system of the present invention.

The ethylenically unsaturated, free radical polymerizable monomers useful in the present invention preferably comprise addition polymerizable, non-gaseous (boiling point above 100° C. at normal atmospheric pressure), ethylenically-unsaturated organic compounds containing at least one, and preferably at least two, terminal ethylenically unsaturated groups, and being capable of forming a high weight average molecular weight polymer by free radical initiated, chain propagating addition polymerization.

The (meth)acrylate component may be selected from

-   -   mono(meth)acrylate monomers comprising formula I:         where G is hydrogen, halogen, or an alkyl having from 1 to 4         carbon atoms, R¹ has from 1 to 16 carbon atoms and is an alkyl,         cycloalkyl, alkenyl, cycloalkenyl, alkaryl, aralkyl, or aryl         group, optionally substituted or interrupted with silane,         silicon, oxygen, halogen, carbonyl, hydroxyl, ester, carboxylic         acid, urea, urethane, carbamate, amine, amide, sulfur,         sulfonate, sulfone or tetrahydrafurfuryl, such as citronellyl         (meth)acrylates, hydroxylethyl (meth)acrylates, hydroxypropyl         (meth)acrylates, tetrahydrodicyclopentadienyl (meth)acrylate,         triethylene glycol (meth)acrylates, and tetrahydrafurfuryl         (meth)acrylates;     -   a di- or tri-(meth)acrylate monomers, such as those selected         from polyethylene glycol di(meth)acrylates, bisphenol-A         di(meth)acrylates, tetrahydrofurane di(meth)acrylates,         hexanediol di(meth)acrylates, polythylene glycol         di(meth)acrylates, such as triethylene glycol di(meth)acrylate,         tripropylene glycol di(meth)acrylate, tetraethylene glycol         di(meth)acrylate, diethylene glycol di(meth)acrylate,         1,4-butanediol di(meth)acrylate, 1,6-hexanediol         di(meth)acrylate, pentaerythritol tetra(meth)acrylate,         trimethylol propane tri(meth)acrylate, trimethylol propane         tri(meth)acrylate, di-pentaerythritol         monohydroxypenta(meth)acrylate, pentaerythritol         tri(meth)acrylate, ethoxylated bisphenol-A di(meth)acrylate,         ethoxylated trimethylolpropane tri(meth)acrylates,         trimethylolpropane propoxylate tri(meth)acrylates, or         combinations thereof;     -   a di(meth)acrylate ester comprising formula II:         where R² is hydrogen, halogen, or an alkyl having about 1 to         about 4 carbon atoms, R³ is hydrogen, an alkyl having about 1 to         about 4 carbon atoms, hydroxyalkyl having about 1 to about 4         carbon atoms or         R⁴ is hydrogen, hydroxy or         and m is 1 to 8, n is 1 to 20, and v is 0 or 1,     -   acrylate esters comprising formula III:         where r is zero or a positive integer, s is zero or 1, t is an         integer from 1 to 20, and p is 2, 3 or 4; G and R⁴ are as         defined above; and R⁵ denotes an organic radical of valency s         linked through a carbon atoms or carbon atoms thereof to the         indicated t number of oxygen atoms. Preferably, r, s, and t are         each 1, G is hydrogen or methyl, and R⁵ is a hydrocarbon residue         of an aliphatic polyhydric alcohol having from 2 to 6 carbon         atoms, such as a pentaerythrityl group. A specific example of         such compounds is pentaerythritoyl tetrakis (dimethylene glycol         acrylate);     -   (meth)acrylate esters comprising formula IV:         where s and p are as defined above, R⁶ is hydrogen or methyl,         and R⁷ is an organic radical of valency p, linked through a         carbon atom thereof other than the carbon atom of a carbonyl         group. More particularly, when s is zero, R⁷ may denote the         residue, containing from 1 to 60 carbon atoms, of an alcohol or         phenol having p hydroxyl groups. R⁷ may thus represent an         aromatic, araliphatic, alkaromatic, cycloaliphatic,         heterocyclic, or heterocycloaliphatic group, such as an aromatic         group containing only one benzene ring, optionally substituted         by chlorine, bromine or an alkyl group of from 1 to 9 carbon         atoms, or an aromatic group comprising a chain of two to four         benzene rings, optionally interrupted by ether oxygen atoms,         aliphatic hydrocarbon groups of 1 to 4 carbon atoms, or sulphone         groups, each benzene ring being optionally substituted by         chloride, bromine or an alkyl group of from 1 to 9 carbon atoms,         or a saturated or unsaturated, straight or branched-chain         aliphatic group, which may contain ether oxygen linkages and         which may be substituted by hydroxyl groups, especially a         saturated or monoethylenically unsaturated straight chain         aliphatic hydrocarbon group of from 1 to 8 carbon atoms.

Specific examples of such groups are the aromatic groups of the formulae —C₆H₄C(CH₃)₂C₆H₄—, in which case p is 2, and —C₆H₄(CH₂C₆H₃—)_(w)—CH₂ C₆H₄—, where w is 1 or 2, in which case p is 3 or 4, and the aliphatic groups of formula —CH₂CHCH₂— or —CH₂CH(CH₂)₃CH₂—, in which case p is 3, or of formula —(CH₂)₄—, —CH₂CH═CHCH₂—, —CH₂CH₂OCH₂CH₂—, or —(CH₂CH₂O)₂CH₂CH₂—, in which case p is 2. When s is 1, R⁷ may represent the residue, containing from 1 to 60 carbon atoms, of an acid having p carboxyl groups, preferably a saturated or ethylenically unsaturated, straight chain or branched aliphatic hydrocarbon group of from 1 to 20 carbon atoms, which may be substituted by chlorine atoms and which may be interrupted by ether oxygen atoms and/or by carbonyloxy (—COO—) groups, or a saturated or ethylenically unsaturated cycloaliphatic or aliphatic-cycloaliphatic hydrocarbon group of at least 4 carbon atoms, which may be substituted by chlorine atoms, or an aromatic hydrocarbon group of from 6 to 12 carbon atoms which may be substituted by chlorine or bromine atoms.

Further preferred compounds where s is 1 include those in which R⁷ represents a saturated or ethylenically unsaturated straight chain or branched aliphatic hydrocarbon group of from 1 to 8 carbon atoms, optionally substituted by a hydroxyl group, or a saturated or ethylenically unsaturated straight chain or branched aliphatic hydrocarbon group of from 4 to 50 carbon atoms and interrupted in the chain by carbonyloxy groups, or a saturated or ethylenically unsaturated monocyclic or bicyclic cycloaliphatic hydrocarbon group of 6 to 8 carbon atoms, or an ethylenically unsaturated cycloaliphatic-aliphatic hydrocarbon group of from 10 to 51 carbon atoms, or a mononuclear aromatic hydrocarbon group of from 6 to 8 carbon atoms. Specific examples of these carboxylic acid residues are those of formula —CH₂CH₂—, CH═CH—, and —C₆H₄— where p is 2. Specific examples of suitable compounds of the preceding formula are epoxy acrylates such as 1,4-bis(2-hydroxy-3(acryloyloxy)propoxy)butane, poly(2-hydroxy-3-(acryloyloxy)propyl)ethers of bis(4-hydroxyphenyl)methane (bisphenol F), 2,2-bis(4-hydroxyphenyl)propane (bisphenol A) and phenol-formaldehyde novolaks, bis(2-hydroxy-3-acryloyloxypropyl) adipate and the methacryloyloxy analogues of these compounds;

-   -   urethane acrylates and ureido acrylates comprising formula V:         where G is as defined above, R⁸ denotes a divalent aliphatic,         cycloaliphatic, aromatic, or araliphatic group, bound through a         carbon atom or carbon atoms thereof indicated at the —O— atom         and —X— atom or group, X denotes —O—, —NH—, or —N(alkyl)-, in         which the alkyl radical has from 1 to 8 carbon atoms, z is an         integer of at least 2 and at most 6, and R⁹ denotes a z-valent         cycloaliphatic, aromatic, or araliphatic group bound through a         carbon atom or carbon atoms thereof to the indicate NH groups.         Preferably R⁸ denotes a divalent aliphatic group of 2 to 6         carbon atoms and R⁹ denotes one of the following: a divalent         aliphatic group of 2 to 10 carbon atoms, such as a group of         formula: —(CH₂)₆—, CH₂C(CH₃)₂CH₂CH(CH₃) (CH₂)—, or —CH₂CH(CH₃)         CH₂C(CH₃)₂CH₂)₂—; or a phenylene group, optionally substituted         by a methyl group or a chlorine atom; a naphthylene group; a         group of formula: —C₆H₄C₆H₄—, C₆H₄CH₂C₆H₄—, or         —C₆H₄C(CH₃)₂C₆H₄—; or a mononuclear alkylcycloalkylene or         alkylcycloalkylalkylene group, such as a         methylcyclohex-2,4-ylene, methylcyclohex-2,6-ylene, or         1,3,3-trimethylcyclohex-5-ylenemethyl group. Specific examples         include 2,4- and 2,6-(bis(2-acryloyloxyethoxy carbonamido)         toluene and the corresponding methacryloyloxy compounds.

The alkylated borohydrides and tetraalkyl boranes used as initiators of polymerization comprise formula VI:

where R¹ is C₁-C₁₀ alkyl, straight chain or branched,

-   -   R² and R³, which may be the same or different, are H, D, C₁-C₁₀         alkyl or C₃-C₁₀ cycloalkyl, phenyl, or phenyl-substituted C₁-C₁₀         alkyl or C₃-C₁₀ cycloalkyl, provided that any two of R¹-R³ may         optionally be part of a carbocyclic ring, and     -   R⁴ is H, D or C₁-C₁₀ alkyl, and     -   M⁺ is a metal ion or a quaternary ammonium ion comprising         formula VII.

The metal ion may be an alkali metal ion such as lithium, sodium, potassium, or cesium, particularly lithium, sodium or potassium, although metallic elements in the second row of the Periodic Table such as barium, magnesium or calcium may be useable, as may transition metals such as copper, iron or cobalt.

The quaternary ammonium ion comprise formula VII below:

-   -   where R⁵-R⁸, which may be the same or different, are C₁-C₁₀         alkyl, C₂-C₁₀ alkenyl, C₃-C₁₀ cycloalkyl, aryl, C₁-C₁₀         alkylaryl, aryl C₁-C₁₀ alkyl or aryl C₁-C₁₀ cycloalkyl, provided         that any two of R⁵-R⁸ may be part of an optionally unsaturated         carbocyclic ring. Aryl is suitably optionally substituted         phenyl, wherein the phenyl ring may be substituted, for example         with C₁-C₁₀ alkyl, particularly C₁-C₆ alkyl, or halo,         particularly Cl, Br or F. Examples of a quaternary ammonium         cation include a tetra C₁-C₁₀ alkylammonium cation, particularly         a tetra C₁-C₅ alkylammonium cation, for example         tetramethylammonium, tetraethylammonium or         tetra-n-butylammonium, or a tri C₁-C₁₀alkylarylammonium cation         wherein aryl is phenyl, substituted phenyl (with phenyl being         substituted as above), or phenyl-substituted C₁-C₁₀ alkyl or         C₃-C₁₀ cycloalkyl. Tetramethylammonium cation (i.e. where R⁵-R⁸         in formula VII are each C₁, alkyl) is particularly suitable.

In one aspect therefore the invention relates to the use of initiators comprising formula VIII below:

-   -   where R¹-R⁸ are as defined above. An alkyl group may suitably         have 1-6 carbon atoms, for instance 2-4 carbon atoms, and may be         straight chain or branched, provided a suitable number of carbon         atoms are available for branching. A carbocyclic ring may be         bridged by the boron atom.

Suitably at least two, and desirably three, and in some instances four, of R¹-R⁴ are C₁-C₁₀ alkyl.

Preferably the alkylated borohydride is of formula VI;

-   M is an alkali metal ion such as lithium, sodium, potassium, or     cesium, particularly lithium, sodium or potassium, although metallic     elements in the second row of the Periodic Table such as barium,     magnesium or calcium may be useable, as may transition metals such     as copper, iron or cobalt; R¹—R³ may suitably be the same or     different alkyl group and each may suitably be a C₁-C₆ alkyl group,     particularly a C₂-C₄ alkyl group. Desirably the three alkyl groups     R¹-R³ are the same group. A cycloalkyl group may suitably be C₅-C₆     group.

Examples of suitable metal salts in which R¹-R³ are the same alkyl group include lithium triethylborohydride, sodium triethylborohydride, potassium triethylborohydride, lithium tri-sec-butylborohydride, sodium tri-sec-butylborohydride, potassium tri-sec-butylborohydride, lithium trisiamylborohydride, potassium trisiamylborohydride and lithium triethylborodeuteride. A particularly suitable example is lithium tri-sec-butylborohydride.

An example of a compound in which R² is an alkyl group and R¹ and R³ are H is lithium thexylborohydride. An example of a compound in which R¹ and R³ form part of a cyclic ring is lithium 9-borabicyclo[3.3.1]-nonane (“9BBN”) hydride.

Desirably at least one of R² and R³ is a C₁-C₁₀ alkyl group or phenyl, provided that not more than one of R² and R³ is phenyl. It will be understood by those skilled in the art that a phenyl group may be substituted in the ring by one or more substituents which do not affect the activity of the compound of formulae VI or VII as a polymerization initiator. Such ring-substituents include C₁-C₁₀ alkyl, for example C₁-C₆ alkyl, particularly methyl.

The above-identified compounds are commercially available from various suppliers such as Aldrich Chemical Co., e.g., under tradenames such as Super-hydride, Selectride, and Super-deuteride, or Callery Chemicals of Evans City, Pa., under trade names such as CalSelect. The commercially available compounds are in flammable solvents, which however are not reactive toward the alkylated borohydrides and tetraalkyl boranes. Generally, THF and diethyl ether—both flammable solvents—are the ones chosen. Other suitable compounds include the metal borohydrides analogous to the boron compounds as described in the Ritter patents such as diisopinocamphenylborane, dicyclohexylborane, and diisoamylborane.

The quantity of alkylated borohydride may suitably be such as to provide 0.01% to 5% by weight, particularly 0.01% to 2% by weight, such as 0.1 to 1% by weight, of boron in the total composition. For ease in handling, the alkylated borohydride is suitably used in a solution of a carrier whose flash point is above 140° F., desirably above 200° F., in a concentration up to about 1.5M, such as about 1M, although the person skilled in the art will select a suitable concentration depending upon the carrier used.

The alkylated borohydrides are placed in a carrier, which may be in the liquid or solid state, provided that when in the solid state, the carrier is capable of being solubilized by some or all of the remaining components in the inventive composition and provided that the flash point of the carrier is above 140 F, desirably above 200 F.

Waxes useful as some or all of the carrier in the present invention include petroleum waxes, vegetable waxes, insect waxes, animal waxes and synthetic waxes. While waxes are generally solid at ambient temperature conditions, they find utility as some or all of the carrier, to the extent they are soluble in or more of the constituents used to make the compositions, or in the compositions themselves.

Petroleum wax includes a range of relatively high-molecular-weight hydrocarbons (approximately C₁₆ to C₅₀), is solid at room temperature, and is derived from higher boiling petroleum fractions. Three basic categories of petroleum-derived or shale-oil-derived waxes include paraffin (crystalline) wax, microcrystalline wax and petrolatum wax.

Paraffin waxes are produced from the lighter lube oil distillates, generally by chilling the oil and filtering the crystallized wax. Paraffin waxes have a distinctive crystalline structure and a melting point range generally between 48° C. (118° F.) and 71° C. (160° F.). Paraffin wax is macrocrystalline and is composed of about 40-90% normal alkanes with a remainder of C₁₈-C₃₆ isoalkanes and cycloalkanes. Paraffin wax is a petroleum-derived wax usually consisting of high-molecular-weight normal paraffins; distinct from other natural waxes, such as beeswax and carnauba wax (palm tree), which are composed of high-molecular-weight esters, in combination with high-molecular-weight acids, alcohols, and hydrocarbons.

Refined paraffin waxes are low oil, or low liquid paraffin, content waxes, generally with an oil content of 1.0 weight percent or less, under conditions prescribed by test method ASTM D 721. Fully refined paraffin waxes generally have even lower oil content of about 0.5 weight percent or less under the same conditions.

Microcrystalline waxes are produced from heavier lube distillates and residua usually by a combination of solvent dilution and chilling. They differ from paraffin waxes in having poorly defined crystalline structure, darker color, higher viscosity, and higher melting points which typically range from 63° C. (145° F.) to 93° C. (200° F.). Microcrystalline waxes contain more branched and cyclic compounds than paraffin waxes and also vary more widely than paraffin waxes in their physical characteristics. Microcrystalline waxes can be somewhat ductile, but are also often brittle and crumble easily.

Petrolatum waxes (CAS Registry No. 8009-0-8) are derived from heavy residual lube stock by propane dilution and filtering or centrifuging. They are microcrystalline in character, semisolid at room temperature and consist predominantly of saturated crystalline and liquid hydrocarbons having carbon numbers greater than C₂₅.

Useful insect and animal waxes include, but are not limited to, beeswax, spermaceti wax, Chinese wax, wool wax, and shellac wax. The major components of beeswax (CAS Registry No. 8012-89-3) are esters of C₃₀ and C₃₂ alcohols with C₁₆ acids, free C₂₅ to C₃₁ carboxylic acids, and C₂₅ to C₃₁ hydrocarbons. Beeswax typically has a melting point of about 60 to 70° C. Spermaceti wax (CAS Registry Nos. 8002-23-1 and 68910-54-3) is derived from the sperm whale and has a melting point of about 42 to 50° C. Chinese wax (CAS Registry No. 8001-73-8) is formed on branches of ash trees (Fraximus chinensis) from the secretion of the coccus insect (Coccus ceriferus). It is a hard wax with a melting point of about 80 to 84° C. Wool wax (CAS Registry No. 68815-23-6) or lanolin wax (CAS Registry No. 68201-49-0) is extracted from sheep's wool and has a melting point of about 36 to 43° C. Shellac wax is obtained from the lac of a scale insect (Coccus lacca) that feeds on certain trees in southern Asia and has a melting point of about 79 to 82° C.

Useful vegetable waxes include, but are not limited to, carnauba wax, candelilla wax, Japan wax, ouricury wax, rice-bran wax, jojoba wax, castor wax, bayberry wax, and soy bean wax. Carnauba wax (CAS Registry No. 8015-86-9) is produced from fronds of a palm tree. The major components of carnauba wax are aliphatic and aromatic esters of long-chain alcohols and acids, with smaller amounts of free fatty acids and alcohols, and resins. Carnauba wax is very hard with a melting point of about 83 to 86° C. Candelilla wax (CAS Registry No. 8006-44-8) is produced from shrubby spurges (Euphorbia antisyphilitica) native to southwest Texas and Mexico. The major components of candelilla wax are hydrocarbons, esters of long-chain alcohols and acids, long-chain alcohols, sterols, and neutral resins, and long-chain acids. Typically, candelilla wax has a melting point of about 67 to 70° C. Japan wax (CAS Registry No. 8001-39-6) is derived from the berries of a small tree native to Japan and China cultivated for its wax. Japan wax is composed of triglycerides, primarily tripalmitin. Japan wax typically has a melting point of about 48 to 53° C. Ouricury wax (CAS Registry No. 68917-70-4) is a brown wax obtained from the fronds of a palm tree which grows in Brazil and has a melting point of about 79 to 84° C. Rice-bran wax (CAS Registry No. 8016-60-2) is extracted from crude rice-bran oil and has a melting point of about 75 to 80° C. The wax is primarily composed of esters of lignoceric acid, behenic acid, and C₂₂-C₃₆ alcohols. Jojoba wax (CAS Registry No. 61789-91-1) is obtained from the seeds of the jojoba plant. Castor wax (CAS Registry No. 8001-78-3) is catalytically hydrogenated castor bean oil. Bayberry wax (CAS Registry No. 8038-77-5) is obtained from the surface of the berry of the bayberry (myrtle) shrub. The wax is made up primarily of lauric, myristic, and palmitic acid esters. The wax has a melting point of about 45 to 49° C.

Useful mineral waxes include, but are not limited to, montan wax, peat wax, ozokerite wax and ceresin wax. Montan wax (CAS Registry Nos. 8002-53-7) is derived by solvent extraction of lignite. The wax components of montan is a mixture of long chain (C₂₄-C₃₀) esters, long-chain acids, and long chain alcohols, ketones, and hydrocarbons. Crude montan wax from Germany typically has a melting point of about 76 to 86° C. Peat waxes are much like montan waxes and are obtained from peat and has a melting point of about 73 to 76° C. Ozokerite wax (CAS Registry No. 001-75-0) was originally a product of Poland, Austria and the former USSR where it was mined and has a melting point of about 74 to 75° C. Ceresin wax (CAS Registry No. 8001-75-0) originally was a refined and bleached ozokerite wax.

Synthetic waxes include, but are not limited to, polyethylene waxes α-olefin waxes, carbowaxes and halowaxes. Polyethylene waxes (CAS Registry No. 8002-72-4) are obtained polymerization of polyethylenes or by Fischer-Tropsch synthesis. The waxes have melting points ranging from about 45-106° C. These waxes may also be chemically modified to vary properties, such as acid number. Polymerized α-olefins can be produced to have waxlike properties and are sold as synthetic waxes. The polymerization process yields highly branched materials, with broad molecular weight distributions. Carbowaxes (CAS Registry Nos. 9004-74-4 and 25322-68-3) are high molecular weight polyethylene glycols. Halowaxes (CAS Registry Nos. 1321-65-9, 1335-87-1, 1335-88-2, 12616-35-2, 12616-36-3, 25586-43-0, 57817-66-7 and 58718-67-5) are chlorinated naphthalenes.

Examples of particularly desirable carriers include generally those selected from certain ethers, epoxies, and hydrocarbons, and more specifically poly(tetrahydofurane), 2-haloalkyl phenyl ethers, such as 2-bromoethyl phenyl ether [flash point, 150° F. (65.3° C.)], 2-chloro ethyl phenyl ethyl, glycidoxy alkyltrialkoxyl silanes, such as 3-glycidoxypropyltrimethoxysilane, certain glycidyl ethers, such as glycidyl heptyl ether, glycidyl undecyl ether, glycidyl ether, glycidyl heptyl ether, propanediol diglycidyl ether, butene diol glycidyl ether, cyclohexane dimethanol diglycidyl ether, 2-ethyl hexyl glycidylether, 1-benzyl-2,3-isopropylidene-S/N-glycerol, and the like. For instance triglyme (flash point, 175° F.) and tetraglyme (flash point, 230° F.) are particularly desirable.

The use of tetraglyme as a carrier for the alkylated borohydrides or tetraaklyl boranes has proven particularly desirably from the standpoint of processing ease and flammability/combustibility reduction.

Carriers useful in the present invention solubilize the metal alkyl borohydride or tetraaklyl borane at the amount intended to be used, is unreactive toward the alkylated borohydrides or tetraaklyl boranes and has a flash point greater than 140° F., desirably greater than 200° F.

The quantity of carrier may suitably be in the range from about 2% to about 50% by weight, for example 2% to 40% by weight, particularly 2% to 30% by weight, of the total composition.

The alkylated borohydrides or tetraaklyl boranes used in the present invention are not contacted with amines, which have been used in the past to form covalent complexes with organoboranes. Once the organoborane of such complexes has reacted in the polymerizable composition, the amine can cause yellowing of a cured composition after aging. A reduction or elimination of yellowing is a particular advantage for adhesives that are to be used in locations where appearance is important, e.g., in automobile manufacture. Primary and/or secondary amines may also give rise to odor problems. Compositions containing the metal alkyl borohydrides have good shelf stability.

It is important that the alkylated borohydrides or tetraaklyl boranes and the carrier do not react together or start to cross-link, causing a change in viscosity, before polymerization of the total polymerizable composition is initiated.

For this reason, the alkylated borohydrides or tetraaklyl boranes and carrier are suitable for packaging together in Part B—the initiator component—of a two-part composition.

In a further aspect of the invention, there is provided an indication system for use in moderating the cure speed of the adhesive system as well as determining when the cure is complete.

More specifically, in this aspect the present invention provides a free radical polymerizable composition, such as one having a (meth)acrylate component, and adhesives based on such free radical polymerizable compositions. Ordinarily, the compositions include a (meth)acrylate component, an alkylated borohydride or tetraaklyl borane, a carrier for the alkylated borohydride or tetraaklyl borane, respectively, in which the alkylated borohydride or tetraaklyl borane, respectively, is soluble, which is unreactive toward the alkylated borohydride or tetraaklyl borane, respectively, and which has a flash point above 140° F., desirably above 200° F., and an indicator system that may control the rate, and indicate the beginning, of cure.

The indicator system is substantially dissolved in the composition, particularly the (meth)acrylate part thereof (Part A), to provide a first color to the (meth)acrylate component, where after mixing with the initiator part (Part B) for a pre-determined amount of time, the composition undergoes a color change as an indication that cure is beginning to occur.

The polymerizable composition may suitably be a two-part composition in which the free-radically polymerizable monomer component is provided in one part and an alkylated borohydride or tetraaklyl borane is provided in the other part. Alternatively, the alkylated borohydride or tetraaklyl borane may be provided as a primer, where the alkylated borohydride or tetraaklyl borane is applied to a substrate separately from the adhesive composition.

In a still further aspect the invention provides a two-part polymerizable adhesive composition, which includes:

-   -   part A—(meth)acrylate component, optionally with an acid.     -   part B—an effective amount of a polymerization initiator         comprising an alkylated borohydride or tetraaklyl borane as         defined herein, and a carrier for which inert to the alkylated         borohydride or tetraaklyl borane as defined herein, where each         part has a flash point greater than 140° F., preferably greater         than 200° F.

Either part A, part B, or both may further include a toughener, filler and/or thickener.

Suitably the compound that is reactive with the alkylated borohydride or tetraaklyl borane as defined herein, is present in part A of a two-part polymerizable acrylic adhesive composition as described above.

Moisture is excluded from contact with the alkylated borohydride or tetraaklyl borane until polymerization has been initiated. Suitably part B of the two-part adhesive composition is packaged in a moisture-free and moisture-impermeable applicator or other container.

The compositions disclosed herein are useful for bonding a large range of substrates including metals, plastics and glass to similar or different substrates and indeed they give better performance than the previously discussed adhesives on many of these substrates, especially plastic. The compositions of the invention are particularly useful for bonding low surface energy substrates, such as those having a surface energy of less than 45 mJ/m², more particularly polyolefins including polyethylene and polypropylene, acrylonitrile-butadiene-styrene and polytetrafluoroethylene, or relatively low surface energy substrates such as polycarbonate, to similar substrates, to each other, or to different substrates including metals, other plastics and glass.

In a particular aspect the invention relates to an adhesive composition as defined above for bonding a low surface energy substrate to a similar or different substrate. In such composition it is preferred that at least two of R₁ and R₂ are straight chain C₁-C₄ alkyl.

Further the invention provides a method for bonding a substrate to a similar or different substrate, where the method comprises applying an adhesive composition as defined above to at least one of the substrates, bringing the substrates together and allowing the composition to cure. In one aspect, the invention provides a method as defined above for bonding a low surface energy substrate such as a polyolefin substrate to a similar or different substrate.

In particular the invention provides a method for bonding a substrate, particularly a low surface energy substrate, to a similar or different substrate, where the method includes mixing parts A and B of a two part adhesive composition prior to use in order to initiate polymerization, applying the mixed composition to at least one of the substrates, bringing the substrates together and allowing the composition to cure by completion of the polymerization initiated on mixing of the parts A and B.

The acid when added may be a weak acid, such as having a pKa no lower than about 0.5 with the desirable limit being about 0.9. The upper limit normally is about 13, or less, such as about 11.5. Carboxylic acids, which have a pKa of up to about 8, such as about 6 or about 7, are particularly useful. An effective amount of the acid, is about 0.1 to about 25%, preferably from about 0.1 to about 20%, and more preferably from about 0.5 to about 10% based on a total weight of the (meth)acrylate composition.

Suitable acids may be monobasic or polybasic. Examples of suitable acids are formic acid, acetic acid, propionic acid, maleic acid, malic acid, fumaric acid, acrylic acid and copolymers thereof, methacrylic acid and copolymers thereof, pyruvic acid, itaconic acid, nadic acid, benzoic acid, phthalic acids, cinnamic acid, trichloroacetic acid, and saccharin.

Lewis acids may also be used. The acid may also be present as a latent acid, particularly a masked carboxylic acid compound hydrolyzable on contact with moisture, such as an acid anhydride, such as described in European Patent No. EP 356 875 and U.S. Pat. No. 5,268,436, the contents of which are incorporated herein by reference.

While alkylated borohydrides or tetraaklyl boranes are effective initiators without an acid, the inclusion of an acid (which term includes a latent acid) together with the free radical polymerizable monomer component is desirable. For certain substrates and/or certain polymerization speeds, the presence of an acid may be required. In the case of a two-part composition, the reaction takes place after mixing of the two parts, one containing the alkylated borohydride or tetraaklyl borane and the other containing the acid or other compound reactive therewith.

The aziridine-containing compound has at least one aziridine ring or group:

where the carbon atom(s) may be optionally substituted with short chain alkyl groups having from one to ten carbon atoms, preferably methyl, ethyl or propyl such that methyl, ethyl or propyl aziridine moieties are formed. Polyfunctional aziridines, such as trimethylolpropane tris(3-(2-methylaziridino) propionate, are particularly useful.

The aziridine-containing compound is preferably soluble in the (meth)acrylate, such that the parts of a multi-part adhesive system can be readily mixed. By “soluble” is meant that no evidence of gross phase separation at room temperature, i.e., about 22° C. to about 25° C., is visible to the unaided eye. Suitably, the aziridine-containing compound is a liquid at or near room temperature, i.e., within about 10° C. of room temperature.

As noted, tougheners may be added. Suitable tougheners include elastomeric materials such as polybutadiene rubbers, polyisoprene, available under the trade name KRATON® from Shell Chemical Corp., Houston, Tex., acrylonitrile-butadiene-styrene (“ABS”), available under the trade name HYCAR®, or as core-shell polymers under the trade name BLENDEX®, or polystyrenes.

For bonding polyolefins, it is desirable to use a core-shell polymer. The use of core-shell polymers in (meth)acrylate compositions is described, for example, in U.S. Pat. Nos. 4,536,546 and 4,942,201, the disclosures of each of which are hereby expressly incorporated therein by reference. Core shell polymers are suitably graft copolymer resins, e.g., ABS graft copolymers, or others described in the '546 and '201 patents, in the form of particles that comprise rubber or rubber-like cores or networks that are surrounded by relatively hard shells. In addition to improving the impact resistance of the bond, core-shell polymers can also impart enhanced spreading and flow properties to the composition. These enhanced properties include a reduced tendency for the composition to leave an undesirable “string” upon dispensing from a syringe-type applicator, or sap or slump after having been applied to a vertical surface. The quantity of toughener, when used, may suitably begin the range from about 1% to about 40%, preferably about 5% to about 25%, based on a total weight of the composition.

Polymeric thickeners may be present in the compositions in an amount up to about 50%, and may be polymers or prepolymers of low or high molecular weight. Suitable polymeric thickeners are a commercially available methacrylate polymer sold under the trademark ELVACITE®, available from E. I. DuPont de Nemours, Wilmington, Del., as well as styrene-methyl methacrylate copolymers, polybisphenol A maleate, or propoxylated bisphenol-A-fumarate polyester sold under the trademark ATLAC®.

Inert fillers, such as finely divided silica, fumed silica (treated or untreated), montmorillonite, clay, bentonite and the like, may also be added. The use of micronized silica can result in a paste-like thixotropic composition. Polymeric thickeners or other thickeners such as silica may suitably be present as a thickener in both Parts A and B of a two-part composition. Inert fillers such as wood flour, cornstarch, glass fibers, cotton lintners, mica, alumina, silica, teflon and the like may be used to alter physical properties, such as modifying viscosity, improving impact resistance, and the like. Such fillers may be incorporated in the inventive compositions in an amount within the range of from about 0.5% to about 20%, for example about 1% to about 10%, by weight of the composition.

It is a particular advantage of the invention that polyolefin and other low surface energy materials can be used as fillers in the composition. Polyolefin powders such as polyethylene powder are relatively inexpensive. In a composition which readily bonds to a polyolefin, the filler becomes securely adhered into the cured composition. Polyethylene or polypropylene powders can be used with particle sizes in the range from about 0.01 microns to about 1 micron, particularly about 0.02 microns to about 0.3 microns, allowing for a good control of the gap between substrates, i.e., the depth of adhesive. Particularly suitable polyethylene powders are commercially available under the trade name MICROTHENE® available from Equistar Chemicals LP, Houston, Tex. Polyethylene flock and polyolefin chopped fibers can also be used as fillers. The amount of polyolefin filler that may be used in the present invention, is about 0.5% to about 20%, preferably about 1% to about 10% based on a total weight of the composition.

Small amounts of silane coupling agents may also be added to increase moisture resistance and well as to enhance bond strength with glass and similar surfaces.

In the two part adhesive system of the present invention, Parts A and B may be provided in a weight ratio in the range from about 1:10 to 10:1, preferably, about 1:5 to 1:1, more preferably from about 1:2 to 1:1, and even more preferably, from about 8:10 to about 10:8. Advantageously, the two part adhesive system is provided in about a 1:1 ratio of Parts A and B for simple consumer use.

Commercially available acrylic adhesive compositions for bonding low surface energy substrates are currently dispensed in a volume ratio of 1:10 (the volume ratios and weight ratios not being significantly different). It is desirable to be able to dispense a two-part composition in approximately equal volumes of each part. This is easier for the end user and has general appeal from the customer point of view.

The composition may optionally include oxidizing agents, reducing agents, thickeners, non-reactive dyes and pigments, reaction indicators (as noted), fire retarders, stabilizers, such as quinones and hydroquinones, thixotropes, plasticizers and antioxidants. The optional additives are used in an amount that does not significantly adversely affect the polymerization process or the desired properties of polymerization products made thereby.

No light-absorbing dyes, cationic dyes, visible light or ultraviolet radical polymerization initiators or hexaarylbiimidazole are required in the adhesive compositions of the invention. In fact, in preferred compositions none are present.

EXAMPLES Example 1

A two part composition in accordance with the invention was prepared as follows:

-   -   Part A was prepared using the following components in the noted         weight percentages: tetrahydrofurfuryl (meth)acrylate, 45%,         2-ethyl hexyl(meth)acrylate, 15%, ATLAC® 22%,         (meth)acryloyoxyethyl maleate, 12.5%, fumed silica (AEROSIL         200), 3.5%, and glass beads (125-150 um), 2%.     -   Part B was prepared by lithium tri-sec-butyl borohydride (1M in         THF) in an amount of 40 parts by weight adding at room         temperature to trimethylol propane tri-3,2-methylaziridine         propionate in an amount of 60 parts by weight. The THF was         removed under vacuum, and to this mixture was added a fumed         silica (HDK-2000) in an amount of 7% by weight. Then, tetraglyme         was added as a carrier in varying amounts to different portions         of the mixture: 11.5%, 23.7% and 32.0% by weight.

Next, Part A was mixed into Part B in a 10:1 ratio and dispensed into aluminum dishes. Polymerization was observed to occur immediately, with full cure confirmed after 3 days.

Example 2

In this example, the same akylated borohydride (1M in THF) was used and tetraglyme was added thereto. THF was removed under vacuum to yield an approximately 1M solution of lithium tri-sec-butyl borohydride in tetraglyme. The aziridine and the fumed silica from above were added with the weight percentages of each component in the Part B as follows:

-   -   Aziridine, 54.38%, borohydride in tetraglyme, 36.25%, and         silica, 9.37%.

The same Part A as above was used. The Part A and the Part B were manually transferred from their mixing vessel, to a 10:1 ratio dual cartridge polypropylene syringe, from which the two parts were dispersed through a static mixer and dispensed onto various low energy substrates.

In order to evaluate the lap shear strength and block shear strength, the area on the substrates to be bonded was cleaned with isopropyl alcohol, the adhesive was applied to one of the substrates through the static mixer, two 0.005″ spacers were placed in the applied bead of adhesive, the second substrate was mated with the first substrate, so as to form a 0.5″ overlap, the substrates with the adhesive therebetween were clamped together, and the assemblies were allowed to cure for 24 hours at ambient temperature and humidity conditions. Some of the assemblies were conditioned at a temperature of 120° F. at 100% realtive humidity (“RH”) for a period of time of 1 week, if indicated.

The shear strength of the bonded assemblies was determined on an Instron, with the peak strength (in psi) and failure modes observed and recorded. The bonded assemblies made from plastic substrates were tested at a shear rate of 0.5″/minute and those made from metals at a shear rate of 0.05″/minute.

The results from five replicate runs are shown below in Tables 1-2. PP is polypropylene; LDPE is low density polypropylene; HDPE is high density polypropylene; PVC is polyvinyl chloride; ABS is acrylo-butadiene-styrene; and PC is polycarbonate.

TABLE 1 Shear Strength Condition- Values (psi) Run Substrate Test ing Mean Std. Dev. % Ret. 1 PP Block shear None 1600 311 — 2 LDPE Block shear None 2348 492 — 3 HDPE Block shear None 2195 271 — 4 PVC Block shear None 1955 522 — 5 PC Block shear None 2145 320 — 6 ABS Block shear None 1331 118 — 7 PP Block shear 120° F./ 1026 325 64 100% RH 8 LDPE Block shear 120° F./ 1300 308 55 100% RH 9 HDPE Block shear 120° F./ 2401 342 109 100% RH 10 Steel Lap shear None 1460 142 — 11 Aluminum Lap shear None 969 140 — 12 Stainless Lap shear None 996 165 — 13 Steel Lap shear 120° F./ 0 0 0 100% RH 14 Steel Lap shear 120° F./ 160 151 18 100% RH

TABLE 2 Run Substrate Test Failure Mode Conditioning 1 PP Block shear Substrate/Adh. None 2 LDPE Block shear Cohesive None 3 HDPE Block shear Cohesive None 4 PVC Block shear Cohesive None 5 PC Block shear Cohesive None 6 ABS Block shear Substrate None 7 PP Block shear Adhesive 120° F./ 100% RH 8 LDPE Block shear Substrate 120° F./ 100% RH 9 HDPE Block shear Cohesive 120° F./ 100% RH 10 Steel Lap shear Adhesive None 11 Aluminum Lap shear Adhesive None 12 Stainless Lap shear Adhesive None 13 Steel Lap shear Adhesive 120° F./ 100% RH

Example 3

Here, Part A from above was used, except the following weight percentages of each component were: THF (meth)acrylate, 48%, 2-ethhylhexyl(meth)acrylate, 16.5%, ATLAC®, 22%, (meth)acryloyloxyethyl maleate, 8%, silica, 3.5% and glass beads (125-150 um), 2%.

The same alkylated borohydride (1M in THF) was used, and the aziridine from above was added. THF was removed under vacuum, and tetraglyme was then added to the mixture. Fumed silica was then introduced with mixing to yield a Part B with following weight percentages of each component; aziridine, 54.38%, borohydride, 7.75%, tetraglyme, 28.5%, and silica, 9.37%.

The flash point of Part B was measured in accordance with ASTM D3278 and determined to be greater than 200° F. The individual components of Part B each have flash points greater than 140° F., and all but tetrahydrafurfuryl (meth)acrylate (195° F.) and 2-ethyl hexyl(meth)acrylate (198° F.) have a flash point greater than 200° F. 

1. A polymerizable adhesive composition for bonding a low surface energy substrate to a similar or different substrate, comprising: a) at least one free-radically polymerizable monomer component, and b) an effective amount of an initiator system for initiating polymerization of the free-radically polymerizable monomer, said initiator system comprising: i) alkylated borohydrides comprising formula VI below:

 wherein R¹ is C₁-C₁₀ alkyl, R² and R³, which may be the same or different, are H, D, C₁-C₁₀ alkyl or C₃-C₁₀ cycloalkyl, phenyl, or phenyl-substituted C₁-C₁₀ alkyl or C₃-C₁₀ cycloalkyl, provided that any two of R¹-R³ may optionally be part of a carbocyclic ring, R⁴ is H, D or C₁-C₁₀ alkyl, and M⁺is a metal ion or a quaternary ammonium ion comprising formula VII below

 wherein R⁵-R⁸, which may be the same or different, are C₁-C₁₀ alkyl, C₂-C₁₀ alkenyl, C₃-C₁₀ cycloalkyl, aryl, C₁-C₁₀ alkylaryl, aryl C₁-C₁₀ alkyl or aryl C₁-C₁₀ cycloalkyl, provided that any two of R⁵-R⁸ may be part of an optionally unsaturated carbocyclic ring; and ii) an aziridine-containing compound; and c) a carrier for the initiator system of b), which renders the composition having a flash point greater than 140° F.
 2. A composition according to claim 1 which is a two-part composition in which the free-radically polymerizable monomer component is provided in one part and component i) of initiator system b) is provided in the other part.
 3. A composition according to claim 1 which is a two-part composition in which the free-radically polymerizable monomer component is at least one (meth)acrylate monomer which is provided in both parts and the initiator system of b)i) is provided in one part.
 4. A two-part polymerizable adhesive composition according to claim 3 comprising: part A) an effective amount of an initiator system of b)i), and a carrier for which inert thereto; and part B) at least one (meth)acrylate monomer, optionally with one or more of a toughener, acid, filler and thickener.
 5. A two-part polymerizable adhesive composition according to claim 3 comprising: part A) an effective amount of component i) of initiator system b), a carrier for which inert thereto, and at least one (meth)acrylate monomer, optionally with one or more of a toughener, filler and thickener, but without any compound that is reactive with the initiator system to free an alkylated borohydride therefrom; and part B) at least one (meth)acrylate monomer together with a compound reactive with an initiator system of b)i) to free an alkylated borohydride therefrom and optionally with on one or more of a toughener, filler and thickener.
 6. A composition according to claim 1 wherein three of R¹-R⁴ of the alkylated borohydride of formula VI are the same C₁-C₁₀ alkyl group and one of R¹-R⁴ is phenyl or phenyl-substituted C₁-C₁₀ alkyl.
 7. A composition according to claim 1 wherein three of R¹-R⁴ of the alkylated borohydride of formula VI are the same C₂-C₄ alkyl group and one of R¹-R⁴ is phenyl.
 8. A composition according to claim 1 wherein each of R⁵-R⁸ of the quaternary ammonium ion of formula VII is the same C₁-C₅ alkyl group.
 9. A composition according to claim 1 wherein the initiator system comprises:

wherein R¹-R³ are each the same C₁-C₆ alkyl group, R⁴ is phenyl; R⁵-R⁸ are each the same C₁-C₆ alkyl group.
 10. A composition according to claim 1 wherein the initiator system is selected from the group consisting of tetra methyl ammonium phenyl triethyl borate and tetra methyl ammonium phenyl tri-n-butyl borate.
 11. A composition according to claim 1 wherein the free-radically polymerizable monomer component comprises a (meth)acrylic monomer.
 12. A composition according to claim 1 wherein the free-radically polymerizable monomer component comprises at least two (meth)acrylic monomers.
 13. A composition according to claim 1 wherein the initiator system is used in an amount sufficient to provide 0.01% to 5% by weight of boron in the total composition.
 14. A composition according to claim 4 wherein the weight ratio of Part A to Part B is in the range from 8:10 to 10:8.
 15. A composition according to claim 1 wherein the aziridine containing compound is trimethylolpropane tris-3-(2-methylaziridino) propionate.
 16. A method for bonding together substrates, at least one of which is constructed of a low surface energy material, the steps of which comprise: applying a composition according to claim 1 to at least one of the substrates, bringing the substrates together with the composition, and allowing the composition to cure.
 17. A method for bonding together substrates, at least one of which is constructed of a low surface energy material, the steps of which comprise: mixing parts A and B of a two part adhesive composition according to claims 3 prior to use in order to initiate polymerization, applying the mixed adhesive composition to at least one of the substrates, bringing the substrates together with the composition, and allowing the composition to cure by completion of the polymerization initiated on mixing of the two parts A and B.
 18. A cured adhesive composition comprising the polymerization product of a composition according to claim
 1. 19. A bonded article comprising two substrates bonded by the polymerization product of a composition according to claim
 1. 20. A bond formed between two substrates by the polymerization product of a composition according to claim
 1. 21. A composition according to claim 1 further comprising a cure indicator.
 22. A composite article comprising a first substrate and a second substrate bonded to the first substrate by an acrylic adhesive, wherein the acrylic adhesive comprises the polymerization product of a composition according to claim
 1. 